(a) Field of the Invention
The present invention relates to a liquid crystal display device including a sensing element.
(b) Description of Related Art
A liquid crystal display (LCD) device includes a pair of panels provided with pixel electrodes and a common electrode and a liquid crystal layer with dielectric anisotropy interposed between the panels. The pixel electrodes are arranged in a matrix and connected to switching elements such as thin film transistors (TFTs) such that they receive image data voltages row by row. The common electrode covers entire surface of one of the two panels and it is supplied with a common voltage. A pixel electrode and corresponding portions of the common electrode, and corresponding portions of the liquid crystal layer form a liquid crystal capacitor as well as a switching element connected thereto constitute a basic element of a pixel.
An LCD device generates electric fields by applying voltages to pixel electrodes and a common electrode and varies the strength of the electric fields to adjust the transmittance of light passing through a liquid crystal layer, thereby displaying images.
A touch screen panel is an apparatus on which a finger or a stylus is touched to write characters, to draw pictures, or to instruct a device such as a computer to execute instructions by using icons. The touch screen panel has its own mechanism to determine whether and where a touch exists and it is attached usually on a display device such as an LCD device. However, an LCD device provided with a touch screen panel has some defects including high manufacturing cost due to the cost of the touch screen panel, low productivity due to a step for attaching the touch screen panel to the LCD device, reduction of the luminance of the LCD device, increase of the thickness of the LCD device.
A recent development is that photosensors including thin film transistors have been incorporated into pixels in an LCD device instead of a touch screen panel. A photosensor senses the variation of light incident on a screen user's finger or the like, to inform the LCD device whether the user's finger, for example, touches on the screen and where the touch is exerted.
However, the characteristics of a photosensor depend on exterior environments such as strength of external light, strength of backlight lamps, temperature, etc., and thus there may be many errors on the light sensing caused by these factors such that the photosensor informs of the presence of a touch that is not actually present or it fails to inform the presence of an actual touch.
In addition, a photosensor in a pixel of a display device and signal lines for transmitting signals to the photosensor may decrease aperture ratio, thereby degrading the image quality.